This invention relates to nanoparticle cadmium sulfide (CdS) fluorescent probes. Preferably, this invention relates to CdS nanoparticles formed in the presence of an amine-terminated dendrimer and/or polyphosphate-stabilized CdS particles both with average diameters or other critical dimensions (CDs) of several nanometers (nm).
There is presently widespread interest in the physical and optical properties of semiconductor particles with average diameters or CdS measured in nanometers. These particles are often called nanoparticles or quantum dots. The optical properties of such particles depends on their size [Martin, C. R.; Mitchell, D. T., Anal. Chem. (1998) 322A-327A].
Such particles display optical and physical properties which are intermediate between those of the bulk material and those of the isolated molecules. For example, the optical absorption of bulk CdSe typically extends to 690 nm. The longest absorption band shifts to 530 nm for CdSe nanoparticles with 4 nm average diameters [Bawendi, M. G.; et al., Annu. Rev. Phys. Chem. (1990) 41, 477-496].
Sizes of nanoparticies are usually measured by average diameters of equivalent spherical particles. For particles that are not at least approximately spherical, the smallest dimension (called critical dimension or CD) is often used. In nanoparticles a large percentage of the atoms are at the surface, rather than in the bulk phase. Consequently, the chemical and physical properties of the material, such as the melting point or phase transition temperature, are affected by the particle size. Semiconductor nanoparticles can be made from a wide variety of materials including, but not limited to CdS, ZnS, Cd3P2, PbS, TiO2, ZnO, CdSe, silicon, porous silicon, oxidized silicon, and Ga/InN/GaN.
Semiconductor nanoparticles frequently display photoluminescence and sometimes electroluminescence. For example see Dabbousi, B. O., et al., Appl. Phys. Lett. (1995) 66(11), 1316-1318; Colvin, V. L., et al., Nature, (1994) 370, 354-357; Zhang, L., et al., J. Phys. Chem. B. (1997) 101 (35), 874-6878; Artemyev, M. V., et al, J. Appl. Phys., (1997) 81(10), 6975-6977; Huang, J., et al., Appl. Phys. Lett. (1997) 70(18), 2335-2337; and Artemyev, M. V., et al., J. Crys. Growth, (1988) 184/185, 374-376. Additionally, some nanoparticles can form self-assembled arrays.
Nanoparticles are being extensively studied for use in optoelectronic displays. Photophysical studies of nanoparticles have been hindered by the lack of reproducible preparations of homogeneous size. The particle size frequently changes with time following preparation. Particle surface is coated with another semiconductor or other chemical species to stabilize the particle [Correa-Duarte, M. A., et al., Chem. Phys. Letts. (1998) 286, 497-501; Hines, M. A., et al., J. Phys. Chem. (1996) 100, 468-471; and Sooklal, K., et al., J. Phys. Chem. (1996) 100, 4551-4555].
There are several examples of fluorescing cadmium sulfide nanoparticles. Tata, et al. use emulsions [Tata, M., et al., Colloids and Surfaces, 127, 39 (1997)]. Fluorescence of CdS nanocrystals have been observed by low temperature microscopy. Blanton, et al. show fluorescence from 5.5 nm diameter CdS nanocrystals with excitation of 800 nm and emission centered around 486 nm [Blanton, S., et al., Chem. Phys. Letts., 229, 317 (1994)]. Tittel, et al. noticed fluorescence of CdS nanocrystals by low temperature confocal microscopy [Tittel, J., et al., J. Phys. Chem. B, 101(16) (1997) 3013-3016].
A 64 branch poly(propylene imine) dendritic box can trap a Rose Bengal molecule (i.e., a polyhalogenated tetracyclic carboxylic acid dye) to allow it to strongly fluoresce since it is isolated from surrounding quenching molecules and solvents [Meijer, et al., Polym. Mater. Sci. Eng., (1995) 73, 123].
While the absorption and emission spectra of nanoparticles have been widely studied, the scope of these measurements were typically limited to using the optical spectra to determine the average size of the particles. There have been relatively few studies of the time-resolved photophysical properties of these particles.
The emission from silicon nanoparticles has been reported as unpolarized [Brus, L. E., et al., J. Am. Chem. Soc. (1995) 117, 2915-2922] or polarized [Andrianov, A. V., et al., JETP Lett. (1993) 58, 427-430; Kovalev, D., et al., Phys. Rev. Letts. (1997) 79(1), 119-122; and Koch, F., et al., J. Luminesc., (1996) 70, 320-332]. Polarized emission has also been reported for CdSe [Chamarro, M., et al., Jpn. J. Appl. Phys. (1995) 34, 12-14; and Bawendi, M. G., et al., J. Chem. Phys. (1992) 96(2), 946-954]. However, in these cases the polarization is either negative or becomes negative in a manner suggesting a process occurring within the nanoparticle. Such behavior would not be useful for a fluorescence probe for which the polarization is expected to depend on rotational diffusion.
The increasing availability of homogeneous sized nanoparticles suggests more detailed studies of their photophysical properties, which in turn could allow their use as biochemical probes. The first reports of such particles as cellular labels have just appeared [Bruchez, M., et al., Science (1998) 281, 2013-2016; and Chan, W., et al., Science (1998) 281, 2016-2018]. CdS particles have also been synthesized which bind DNA and display spectral changes upon DNA binding [Mahtab, R., et al., J. Am. Chem. Soc. (1996) 118, 7028-7032; and Murphy, C. J., et al., Proc. Materials Res. Soc. (1997) 452, 597-600].
U.S. Pat. No. 5,938,934 to Balogh et al., describes use of dendrimers as hosts for many materials including semiconductors. However the nanoparticles are somewhat large for use as a probe based on size. Only example 15 discloses cadmiums sulfide. However dangerous sulfide gas is used over prolonged periods of time.
This invention describes fabrication methods, spectroscopy, probes and other applications for semiconductor nanoparticles. The preferred embodiments are two types of cadmium sulfide (CdS) nanoparticles. CdS nanoparticles formed in the presence of an amine-terminated dendrimer show blue emission. The emission wavelength of these nanoparticles depends on the excitation wavelength. These CdS/dendrimer nanoparticles display a new constant positive polarized blue emission. Polyphosphate-stabilized CdS nanoparticles are described that display a longer wavelength red emission maximum than bulk CdS and display a zero anisotropy for all excitation wavelengths. Both nanoparticles display strongly heterogeneous intensity decays with mean decay times of 93 ns and 10 xcexcs for the blue and red emitting particles, respectively. Both types of nanoparticles were several times more photostable upon continuous illumination than fluorescein. In spite of the long decay times the nanoparticles are mostly insensitive to dissolved oxygen but are quenched by iodide. These nanoparticles can provide a new class of luminophores for use in chemical sensing, DNA sequencing, high throughput screening, fluorescence polarization immunoassays, time-gated immunoassays, time-resolved immunoassays, enzyme-linked immunosorbent assay (ELISA) assays, filtration testing, and targeted tagging and other applications.